1. You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
    Dismiss Notice

148gtl mic shielding

Discussion in 'CB Radio Modifications' started by 338_MtRushmore, Jun 22, 2018.

  1. 338_MtRushmore

    338_MtRushmore Well-Known Member

    Joined:
    Jun 17, 2012
    Messages:
    767
    Likes Received:
    282
    I added a 4 pin mic jack on the opposite side of the 5 pin. I haven't had any problems with it, but I was thinking about shielding the audio wire between the two jacks. Would it matter what I used for coax, and what would be the proper way to ground it? Just ground both ends to each Mic jack? I could just leave it unshielded I suppose, but it just doesn't seem right.


     

  2. 338_MtRushmore

    338_MtRushmore Well-Known Member

    Joined:
    Jun 17, 2012
    Messages:
    767
    Likes Received:
    282
    Based on the lack of replies, I assume there is no need to fix it if it isn't broke. With my luck I would ground the shield and create a ground loop somehow that would make it squeal like a pig or something. Maybe it was just a retarded question, but I will still leave it alone.
     
  3. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

    Joined:
    Apr 23, 2018
    Messages:
    973
    Likes Received:
    649
    There are a lot of people that have varying opinions and so I don't want to interfere with the due process - but I'll throw my thoughts in here to help you.

    Well, I too get a bit busy, and still am, so forgive the delay in posting.

    Ok, you have a 5-pin radio but a 4-pin setup...(let me get this straight) Opposite side? Wow...kinda' wild - why not just keep the original hole and use it for the 4-pin...?

    Alrighty, tell me when you have time...

    But when you "swap" those 4-pin to 5-pin and back from 5-pin to 4-pin - the grounds for, say, the MB3756 versus the Audio return are in different locations. Which pin you use - may have a trickle voltage when the radio is unkeyed.

    Depending on where - (which pin) you route your ground, what you'll get in noise, is what you're asking - it's not an easy question.

    If you can, make the radio a NEGATIVE Ground Only chassis by soldering a braid wire from the TOP of either your power choke transformer or the metal screw-lug combo used for the bypass caps - to BOND the negative lead at the UNIDEN-Style power connector to the Chassis and make FOIL ground the SAME as Chassis ground. A.K.A. - Strapping the radio. (You can simply unsolder the "Black wire" from the Uniden plug and resolder it past, across from - the bypass filter disc caps - to the solder lug used to solder the NEGATIVE CHASSIS side of that Negative Lead power filter cap that uses the "screw nut bolt combo" on the back of the Uniden Power Plug to complete the circuit just make sure its' the Chassis side of the cap...)

    Then once you've got the chassis and foil ground together, you can use Pin 2 shield ground for this 4-pin plug idea...

    If you don't bond the chassis and foil grounds together, then you will pick up noise and the radio may become unstable (squeal).

    Pin 4 of the 3,4 and 5 Pin combo - Toggle ground - is the UNFILTERED foil board ground for the MB3756 switch function.- and is why the 5-pin DIN idea kept AUDIO and SWITCH functions separate - the 3756 didn't have issues, the trickle voltage from it's lines that goes to TX and RX - has a residual charge from their respecting outputs where noise can be INJECTED into the power regulator power feed lines - and is what made noise in the Mic's own Audio line.

    You also had SPEAKER return too (pin 3 is from speaker) - so once the bonding is done - speaker potential differences as well as grounding for Chassis will be equalized and less likely to cause problems.

    As personal preference - it's up to you - but once the radio is BONDED/Strapped - the issue becomes more of preventing a ground loop from forming and adding more noise - so If Pin 2 doesn't work - use Pin 4 instead.

    And if Alternator whine shows up - then the radio needs to have SINGLE POINT grounding - get rid of the power plugs Negative Wire - cut it off - remove it - use the chassis bolts to bracket, to ground for mobile mounting . Using single wire grounding lets the chassis ground handle the eddy current that will form when you're Negative lead wire is at a different location than the radios own ground. Your radio has a filter "ring" al along the chassis board mounting locations - works well for this - use it.

    Got a Permanent Antenna mount? Good ground to it? - Low SWR too? Fantastic - let that location serve as your single point ground if you can't use the bracket. Just hook up the coax and away you go!

    :+> Andy <+:
     
  4. 338_MtRushmore

    338_MtRushmore Well-Known Member

    Joined:
    Jun 17, 2012
    Messages:
    767
    Likes Received:
    282
    I actually tried several times to just swap the 5 pin for a 4 pin, but had squealing issues. It worked fine with an adapter, but there isn't enough room to clear my sun visor. Just for kicks I wired in a 4 pin to the back of the pins on the 5 pin and it worked. I don't recall which ground I used. I figured at that point I may as well use the hole on the other side of the chassis and kill two birds.

    I am not actually having problems, but it just seems bad to have an unshielded audio wire running across the radio. The good news is, now I know what makes a 5 pin a 5 pin, and what causes the feedback on some radios.
     

Share This Page

  • About Us

    The WorldwideDX Radio Forum was originally established in 2001. We pride ourselves on welcoming Radio Hobby enthusiasts of all types, while offering unbiased, informative, and friendly discussion among the members. We are working every day to make sure our community is the best Radio Hobbyist's site.
  • Like us on Facebook

  • Premium VIP Member

    The management works very hard to make sure the community is running the best software, best designs, and all the other bells and whistles. Care to buy us a beer? We'd really appreciate it!

    Donate to us!